Cylindrical drum for stencil printing

Abstract: In a cylindrical drum (1) for stencil printing which consists of a porous cylindrical plate (2) and a screen layer (3), in which a stencil sheet (4) is attached to the outer surface of the screen layer having an ink permeability, an ink is supplied (10-13) from the inner surface of the porous cylindrical body which rotates around its own central axis, the present invention provides a screen layer consisting of a woven fabric of conjugated fibers of a sheath-and-core or a side-by-side type consisting of a lower melting component and a higher melting component and making the intersections of the fibers fixed together through the melt-adhesion of the lower melting point component by means of thermocompression bonding.

Description: The present invention relates to a cylindrical drum for stencil printing. Specifically, it relates to a cylindrical drum for stencil printing which is suitable for controlling ink transferability, etc., in a rotary type stencil printing machine. A stencil printing method has an advantage in that a plurality of printed copies can thereby be obtained more economically in comparison with the costs of other printing methods. A rotary type stencil printing machine is known as a printing machine which is allowed to exert its advantage at maximum. The rotary type stencil printing machine has a porous cylindrical drum which rotates around its own axis. A stencil sheet is attached to the outer surface of the porous cylindrical drum; an ink being supplied from the inner surface of the porous cylindrical drum. As a cylindrical drum, there is known a drum, such as that which generally has a metallic supporting cylinder having numerous small pores, which may termed a porous cylindrical body, and a screen layer rolled around the outer surface of the supporting cylinder described above, that which directly rolls the screen layer cylindrically around a pair of flanges which are supported by a center rod without using the metallic supporting cylinder described above, and others. As a screen layer described above, in order to improve the ink transfer quality, a screen layer consisting of polyester fibers having a fine net structure, and a screen multi-layer composed of a lower mesh stainless screen and a higher mesh polyester fiber screen are known (Japanese Patent publication No. 63-59393 and Japanese Patent Application laid-open No. 3-254986). However, when the screen layer of a fine net structure is provided to the supporting cylinder, stitch deviation of the screen occurs readily. As a result, there is a disadvantage in that the small pores in the supporting cylinder appear as a shadow on the printed image. Further, in the case of a screen multi-layer, it is often inconvenient that for ink to be retained between the two layers. Also, in the drum having no supporting cylinder, since an ink supply roller built in the cylindrical drum is brought into direct contact with the screen, there are such problems that the printing ink cannot uniformly be supplied because the stitch deviation of the screen is large, the deflection thereof is easily yielded at the time of printing operation because the rigidity thereof is low, and the pressure distribution at the time of printing becomes non-uniform because the surface condition of the screen is not smooth. EP-A-0439960 discloses a mesh fabric for screen printing obtained by use of sheath-and-core type composite filaments in which a component with lower melting point is used as the sheath. The warp and weft threads are adhered to each other at the intersecting points of the threads by melting the sheath component after fabrication. It is accordingly a main aim of the present invention to solve the disadvantages in the prior art and provide such a cylindrical drum for stencil printing that allows low cost operation, and improves the ink transferability and the printing quality. The present invention in one aspect provides a cylindrical drum for stencil printing which comprises: a porous cylindrical body having a means for feeding an ink onto the inner surface thereof; and a screen layer having an ink permeability provided on the outer surface of said porous cylindrical body; characterized in that said screen layer is composed of a fabric woven by using conjugated fibers of a side-by-side type consisting of a lower melting point component as one side component and a higher melting point component as the other side component, the intersections of which fibers are adhered with each other by melt-adhesion of softening of said lower melting point component. Preferably, the means for feeding an ink is a squeeze roller having an ink feeding means, provided in the cylindrical body so as in use to rotate in contact with the inner surface of the cylindrical body. Preferably, the melting point difference between the lower melting point component and the higher melting point component is 20°C or more. Preferably, the higher melting point component of the conjugated fiber is homopolyester and the lower melting point component of the conjugated fiber is copolymerized polyester. Preferably, the intersections of the conjugated fibers in the fabric have been adhered with each other by thermocompression at such temperature and pressure that the lower melting point component is melted. In another aspect the present invention further provides a cylindrical drum for stencil printing which comprises: a pair of flanges supported by a member provided between the flanges; a screen layer having an ink permeability rolled around the pair of flanges to form a cylindrical body; and a means for feeding an ink onto the inner surface of the screen layer; characterized in that said member is a center rod and that said screen layer is composed of a fabric woven by using conjugated fibers of a side-by-side type consisting of a lower melting point component as one side component and a higher melting point component as the other side component, the intersections of which fibers are adhered with each other by melt-adhesion or softening of said lower melting point component. Preferably, the means for feeding an ink is a squeeze roller having an ink feeding means, provided in the cylindrical body of the screen layer so as in use to rotate in contact with the inner surface of the cylindrical body. Preferably, the melting point difference between the lower melting point component and the higher melting point component is 20°C or more. Preferably, the higher melting point component of the conjugated fiber is homopolyester and the lower melting point component of the conjugated fiber is copolymerized polyester. Preferably, the intersections of the conjugated fibers in the fabric have been adhered with each other by thermocompression at such temperature and pressure that the lower melting point component is melted or softened. The screen to be used for the screen layer in the present invention can be obtained by weaving side-by-side type conjugated fibers consisting of both a lower melting point component and a higher melting point component by a conventional method, then subjecting it to thermocompression so as to adhere the intersections of the fibers with each other through the lower melting point component of the fibers thereby. The temperature difference between the melting points of the lower melting point and higher melting point components is preferably 20°C or more. As a lower melting point component of the conjugated fibers, there is no particular limitation of the component so long as the component can bond the intersections of the fibers to each other by being melted or softened through thermocompression. The following thermoplastic resins may be, for example, polyethylene terephthalate (polyester) copolymer, polypropylene, polypropylene-ethylene copolymer, etc. Polyester copolymer can be obtained by copolymerizing the other monomers or reactive components than those used as raw materials at the time of polycondensation of ethylene glycol and phthalic acid. Such monomers or reactive components may be polyalkyleneglycol, dicarboxylic acid, lower molecular weight glycol, etc. As a higher melting point component, there is no particular limitation of the component so long as the component has a good adhesivity to the lower melting point component and does not melt or deform at the time of thermocompression, but it is preferable to use a resin component having a low affinity with the ink in order to improve the permeability of the ink. As such resins, polyethylene terephthalate, polypropylene,etc. are exemplified. Polyethylene terephthalate may be preferably used from the viewpoint of a melting point and strength. In the case when the higher melting point component is polyethylene terephthalate, polyester copolymer is preferred as a lower melting point component. The content ratio of the lower melting point component in the conjugated fibers may be such an amount as the component may be melted for allowing to adhere and fix the intersections of the fibers and not damage the pores in the screen, and it is assumed to be in the range of 5 - 70% and preferably in the range of 10 - 50%. The sectional shape of the conjugated fibers may be a round or a deformed section. The conjugated fibers of side-by-side type can be obtained by a conventional melt-spinning process using known conjugate spinning nozzles and the resulting conjugated fibers (filaments) are woven to a fabric, a plain weave fabric, for example, by a conventional method to obtain the screen layer in the present invention. The screen may be composed only by the conjugated fibers. However, a portion of the fibers may be replaced by regular fibers. For example, the conjugated fibers may be used only for warps or wefts or every other or third of a warp or weft, etc. As a fiber to be used other than the conjugated fibers, for example, regular fibers consisting of polyester having its higher melting point component described above can be used. According to the thermocompression processing, the lower melting point component of the conjugated fibers is melted and the intersections of the fibers are bonded so as to be fixed to each other. The temperatures and pressures for thermocompression bonding are appropriately determined depending on the materials of the screen. For example, in the case of the fabric consisting of polyester conjugated fibers which use copolymer polyester as a lower melting point component and homopolyester as a higher melting component, it can be thermocompressed by making it pass between a metal roller heated at 120°C and a silicone rubber roller under a nip pressure of 1.8 kg/cm 2 . It is not necessary for the temperature of a heating roller to reach the melting point of the lower melting point component as far as the intersection of the fibers are bonded by the lower melting point component. Whether they are bonded or not can be easily observed by microscope, for example. There is no particular limitation of the sieve opening in the screen after the thermal compression bonding, but from the viewpoint of the ink transferability, the range of 70 - 400 mesh is preferable, and its thickness in the range of 40 - 200 µm is preferable. As a screen layer, a single layer is usually preferable, but double layers may be possible. By using the screen layer bonded at the intersections of the fibers, the occurrence of stitch deviation and deflection of the screen can be prevented, the thickness of the screen can be uniformly thinned, and the smoothness in the screen surface can be improved. As a result, whether a supporting cylinder may be available or not, the control of ink transferability becomes easy and the printed image quality as well as the printing workability is improved. Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: - Fig. 1 is a schematic view showing a rotary type stencil printing machine, equipped with a cylindrical drum having a porous cylindrical body, of the present invention. Fig. 2 is an enlarged plan view of a part of the porous cylindrical body constituting the cylindrical drum in Fig. 1. Fig. 3 is a plan view of a screen layer used in the present invention. In Fig. 1, a cylindrical drum 1 consists of a porous cylindrical body 2 having numerous small pores constituting the innermost layer, and a screen layer 3 rolled around the cylindrical body 2. In the cylindrical body, there is provided a squeeze roller 10 which is rotationally driven in the same direction as the cylindrical body 2 while contacting with the inner surface of the cylindrical body 2, and a fixed doctor rod 11 facing the outer surface of the squeeze roller 10 while keeping a predetermined small space 12 between the squeeze roller 10 and the doctor rod 11, and working together with the squeeze roller 10. The cylindrical body 2 and the squeeze roller 10 have each driving means (not shown) for rotation around their center axis. The cylindrical body 2 has numerous ink permeable small pores as illustrated in Fig. 2. The small pores of the supporting cylinder 2 are normally in the range of 20 - 60 mesh. The screen layer 3 consisting of a plain weave fabric is shown in Fig. 3. A press roller 14 for pressing a printing paper 5 to the outer surface of the cylindrical drum 1 is positioned under the cylindrical drum 1 and provided with a mechanism for moving upwardly or downwardly for pressing or releasing the printing paper 5. In the stencil printing, a stencil sheet 4 is attached to the outer surface of the screen layer 3. A printing paper 5 for stencil printing is fed between the cylindrical drum 1 and the press roller 14 and is pressed to the stencil sheet 4 attached to the outer surface of the cylindrical drum 1 by the press roller 14. An ink is supplied to an ink reservoir portion 13 formed between the squeeze roller 10 and the doctor rod 11. The ink in the ink reservoir portion 13 is passed through the squeeze roller 10, porous cylindrical body 2, screen layer 3 and the stencil sheet 4, and transferred onto a printing paper. The feeding amount of ink can be controlled by changing the clearance between the squeeze roller 10 and the doctor rod 11. The details of the present invention will be explained according to the following Examples. It should be understood, however, that the scope and effect of the present invention is not limited by the following examples. Example 1 A screen (sieve opening 200 mesh and thickness 75 µm) was prepared by subjecting side-by-side type polyester conjugated fibers as a weft, which fibers were obtained by conjugate-spinning a lower melting component (copolymerized polyester) and a higher melting component (homopolyester) at the ratio of 50/50 (by weight), and regular polyester fibers as a warp, to a conventional plain weaving process. The resulting screen was thermocompressed by passing it between a metal roller heated at 120°C and a silicone rubber roller at a nip pressure of 1.8 kg.cm 2 . The resulting screen was installed to the drum (a porous cylindrical body) of a stencil printing machine (Riso Kagaku Corporation product, RC-115) to carry out stencil printing in the same apparatus. As a result, a good image was obtained and there were no problems such as the stitch deviation and deflection of the screen during the printing operation. Comparative example 1 A screen (NBC Industry Co. product, PP200, sieve opening 200 mesh and thickness 160 µm) consisting of polypropylene fibers was thermocompressed under a similar condition to that of Example 1, and then, similarly installed to the drum. Stencil printing was thereby carried out. As a result, although the rigidity of the screen was improved, as the some fibers were deformed or melted by heat, the opening ratio of the screen was lowered, the image concentration of the printed matters was extremely thin, and no good images could be obtained. Comparative example 2 A screen (a plain weave fabric made by NBC Industry Co., sieve opening 200 mesh and thickness 75 µm) consisting of polyester conjugated fibers (monofilaments) having a sheath and core structure (sheath component: copolymerized polyester of m.p. ca. 200°C; core component: homopolyester of m.p. ca. 265°C; conjugate ratio (weight) 1:1) was provided. The resulting screen was installed to the drum (a porous cylindrical body) of a stencil printing machine (Riso Kagaku Corporation product, RC-115) to carry out stencil printing in the same apparatus. As a result, stitch deviation and deflection of the screen occurred and non-uniform portions were generated in the printed images. According to the present invention, by using a screen layer bonded at the intersections of the conjugated fibers by thermocompression, it is possible to improve the rigidity and toughness of the screen, prevent the generation of the stitch deviation and deflection of the screen at the time of printing, uniformly reduce the thickness of the screen, improve the smoothness of the screen surface, easily control the ink transferability, and develop the image quality of the printed matters as well as the printing workability. Furthermore, everything may be set up by mounting only a single sheet of the screen layer on the cylindrical body or on the flanges, resulting in a lowering of the cost for assembling the stencil printing apparatus.

Claims: A cylindrical drum (1) for stencil printing which comprises: a porous cylindrical body (2) having a means for feeding an ink onto the inner surface thereof; and a screen layer (3) having an ink permeability provided on the outer surface of said porous cylindrical body (2); characterized in that said screen layer (3) is composed of a fabric woven by using conjugated fibers of a side-by-side type consisting of a lower melting point component as one side component and a higher melting point component as the other side component, the intersections of which fibers are adhered with each other by melt-adhesion or softening of said lower melting point component. A cylindrical drum according to claim 1, wherein said means for feeding an ink is a squeeze roller (10) having an ink feeding means, provided in said cylindrical body (2) so as in use to rotate in contact with the inner surface of said cylindrical body (2). A cylindrical drum according to claim 1 or claim 2, wherein the melting point difference between said lower melting point component and said higher melting point component is 20°C or more. A cylindrical drum according to any one of claims 1, 2 or 3, wherein said higher melting point component of the conjugated fiber is homopolyester and said lower melting point component of the conjugated fiber is copolymerized polyester. A cylindrical drum according to any foregoing claim, wherein said intersections of the conjugated fibers in said fabric have been adhered with each other by thermocompression at such temperature and pressure that the lower melting point component is melted. A cylindrical drum for stencil printing which comprises: a pair of flanges supported by a member provided between the flanges; a screen layer (3) having an ink permeability rolled around the pair of flanges to form a cylindrical body; and a means for feeding an ink onto the inner surface of the screen layer (3); characterized in that said member is a center rod and that said screen layer (3) is composed of a fabric woven by using conjugated fibers of a side-by-side type consisting of a lower melting point component as one side component and a higher melting point component as the other side component, the intersections of which fibers are adhered with each other by melt-adhesion or softening of said lower melting point component. A cylindrical drum according to claim 6, wherein said means for feeding an ink is a squeeze roller (10) having an ink feeding means, provided in said cylindrical body of the screen layer (3) so as in use to rotate in contact with the inner surface of said cylindrical body. A cylindrical drum according to claim 6 or claim 7, wherein the melting point difference between said lower melting point component and said higher melting point component is 20°C or more. A cylindrical drum according to any one of claims 6 to 8, wherein said higher melting point component of the conjugated fiber is homopolyester and said lower melting point component of the conjugated fiber is copolymerized polyester. A cylindrical drum according to any one of claims 6 to 9, wherein said intersections of the conjugated fibers in said fabric have been adhered with each other by thermocompression at such temperature and pressure that the lower melting point component is melted or softened.

Inventors: IKEZIMA SYOUICHI, SUGAYA KENGO RISO, YAMANAKA TETSU, IKEZIMA, SYOUICHI., SUGAYA, KENGO, YAMANAKA, TETSU, IKEZIMA, SYOUICHI, RISO KAGAKU CORP., SUGAYA, KENGO, RISO KAGAKU CORP., YAMANAKA, TETSU, RISO KAGAKU CORP.
Applicants: RISO KAGAKU CORP, RISO KAGAKU CORPORATION
Application year: 1994
Main classification: B41C1
IPC codes: B41L13, B41C1, B41N1